MTB front suspension sounds like rusty trampoline
I think you answered the "cause" part of your question already - "long-term lack of maintenance".

Whether you want to attempt the rehabilitation yourself or not depends on your degree of comfort with that kind of thing. Here is a copy of the manual for the 2001 model (PDF file; make sure it actually matches what you have), which should help you get a sense of what's involved.

Just be glad they're not melted-down elastomer suspensions from the '90s like the last bike I inherited... yeeesh. :P