The watch strap is stuck inside the watch, what should I do?

Please see Apple Support docs

https://support.apple.com/HT204818

https://support.apple.com/HT204522

Note that some users have found the release button was blocked but it released when flushed with warm (not hot) water to dissolve deposits around the button. Whether you can do this will depend on the band material, because water may mark a leather band.

If the band is a third party product (non-Apple) it may not be manufactured to the same precision, and it might be a very tight fit.

I have also found that using a "used" soft bristle toothbrush with a bit of warm water is helpful in dislodging some of the gunk and grime that may be holding the strap in.