What to do when you feel stuck in your career?

TL;DR: Make a list of goals, genuinely connect with people who've accomplished these goals, take actions toward getting unstuck ASAP.

I've been struggling with this recently. Here's the process I've been using to get unstuck. Before making any decisions, find time to reflect on what you want to accomplish and where you want to go. I agree with @ben that thinking in terms of 10+ years is helpful. I'd also encourage you to create short-term goals you envision accomplishing within 3 months - 5 years. Write each of these goals down. Then evaluate where you are in the present and answer this question:

Will the path I'm on allow me to achieve any of these goals?

Because OP's question very pointedly uses the word "stuck", let's assume you're at least partially off-track. Identify which goals are within reach (if any) and then do the same for the goals you aren't currently capable of achieving.

Once you have this list separated, find people who have accomplished the goals you aren't on pace to attain and talk with them. I'm continually shocked by the generosity of people when I reach out via cold email or through virtual communities like the lovely site we're on right now and Indie Hackers. People are generally willing to share their stories and advice with you if you ask. Most people miss the key element of these interactions. Be genuine.

Networking has become a little sleazy. Many people have been trained to view humans as assets in the networking context. If you aren't genuinly interested in talking with people for the sake of learning what they do and why they do it and you're only looking to connect to see what they can do for you, your professional interactions and relationships will always be limited. Those transactional and selfish networking ploys are easy to spot. If that's the type of person you are then you'll benefit from reflecting on why you value purely transactional networking.

Not everyone you reach out to will respond but some will. Listen intently to what they have to say. Ask questions, run ideas by them, get their opinions about what you're thinking and if they've had a similar experience where they've felt stuck. After compiling all of this information start taking action. You're now equipped to make informed decisions!

Presumably you can course correct now. Start looking for a new job, begin that side project, learn a new programming language, adjust your behavior to get that promotion at your current job, quit and start your own company. Whatever the answer is, don't be afraid to exercise the control you have over your career. We're talking about very precious time that shouldn't be wasted.

Learn new technologies/skills and improve existing, work on some side projects they are great for honing your skills, and all the while scout for a better company if you feel stuck. Don't worry an opportunity will arise and because you have been a busy bee in the meantime, you'll be able to act upon it! Some people call it luck, but what actually happens is preparation meats the opportunity ;) No work goes in vain, something good always has to come out of it, so keep yourself busy, no slacking!

Maybe you need some space to figure out what's next, or to decompress, or self-reflect, or whatever.

If you can afford it, it's okay to not work for a while. Add up your expenses and see how long you could last on your savings (be sure to look up rates for insurance and the like, if you live somewhere with a third-rate public health system). Nobody stops you from not working. It does get harder to relate to family and friends though, without a canned "oh, you know, work is work" answer to what you've been up to.

"Self-funded sabbatical" is how I'm calling this period on my resume. YMMV. I'd recommend coming up with a default exit plan for getting back to work with some buffer.

Or maybe you can negotiate a sabbatical from your current job with the expectation that you'll come back afterward. Or cover your expenses some other way. ("consulting" is a thing I hear people say, but it sounds like something for someone with more business skills and gravitas than me.)

Edit to add: That's what I did, but I guess it depends how exactly you're stuck.

I took time off. Parental leave 7 months with my daughter.

This caused my teams to become more independent and I could focus on other stuff when I came back.

I have also not been afraid to make lateral jumps into maybe uncomfortable and less well compensated positions.

Be agile and unafraid!

Think longterm: "where will I be in ten years, it probably doesn't matter if I'm stuck for a little while now"

But don't worry longterm: "things will work out if I take it one day at a time"

Be honest with yourself about what you really want to do with your life, give yourself realistic milestones and start working towards it today, do little bit every day, but don't give up.