Antonym of "suspend" as in "Suspend a user account"

I would use reinstate:

to put back or establish again, as in a former position or state: to reinstate the ousted chairman.

I would use either "resume", "restore" or "reactivate".

"Reactivate" is the clearest, but some may find it ugly.

The others will only make sense in a context where it is clear that the account has been suspended, whereas "reactivate" is fairly obvious even without context.

As a sysadmin the word we usually use is just 'activate'. e.g your account was suspended because the password expired, I have just activated it for you. It may not be grammatically correct but that is usually the on-screen option, button or command used in the software to do what you are saying.

(Or if you want to keep looking for other words, the origin of 'account' comes from mainframes when you had to pay to use it and your account was the amount of money you had put in. When you ran out of money your account was suspended, just like business accounting systems would do to customers. Try looking at business accounting terms if you want more ideas )

If you see "suspend" as a synonym to "lock" because you forbid the person to use the computer system or software he wants, you could use "unlock" as the antonym.