chances are that water has gotten inside the fork and it is now rusted in place. Tip the bike upside down, walk away for a while, and see if anything leaks out of the fork. do this where the leaking stuff WON'T stain anything... it will be black water and grease mixed in it.... it usually stinks, too. I toss down an old towel to catch the gunk.

If any of the control cables are stuck, that's another sign that the bike has been out in the rain... Replace those cables and the chain is usually funky too. Cables are cheap compared to emergency room visits

i have gotten some old stuck forks to move again, but they weren't in what i'd say was usable condition..
IF you can get them to move, then you might be able to take them apart and clean all the gunk out, re-grease, then slip them back together.

Youtube has a bunch of fork cleaning vids...

take a few pics of the bike and fork, then post them here... it will help a lot there's about a thousand different forks., half of which are probably low end Rock Shox or RST's, with either Elastomers (BAD! Usually No Kits Available) or Springs (Better for you, and Rebuildable, USUALLY) in them.

1" or 1 1/8" steerer tube, threaded, or non-threaded steerer tube, sealed oil damped, or vented(grease lubed), etc.Some require some fairly specific tools to work on them properly...
Someone here will recognize which style you have.

And if it is an Elastomer (AKA: Poly Inserts) fork and you FIND kits,. let the world know... i have three gorgeous Answer/Manitou forks, and at least two cool old magnesium Rock Shox tapered forks here, just begging for some new bouncy stuffings!!!