Reactivating a desktop computer that has been off for more than a year.

I have a desktop computer that has been off for well over 4 years (I have been traveling and it has been in storage). I have turned it on, (it will still do that), but it seems stuck in the loading process. It has been well over two hours, the bar has reached the end, and the unit is very warm. If it does not progress from here, what should/can I do? I can't remember what system is running on it, or any information about it except that is is more than 10 years old, (I can't even remember exactly how old it is, older than 2017 I think).