My 390 E is locked up and I can't move the pedals. I checked the belt, it's still on and it's tight.

See torsion spring (17) in the followings JD parts diagram.

https://partscatalog.deere.com/jdrc/sidebyside/equipment/65427/referrer/navigation/pgId/183947567

Check rod (14) is attached from the pedal shaft to the transmission.

Your belt was probably not placed over the tension pulley. Follow the pedal you push to make tractor go forward. Look under the tractor where the belt was installed. If space is tight just lower your deck. The linkage from the pedal will go to the tension pulley. If the belt has been placed on one side of the pulley, and machine is not working then move the belt to the other side of the pulley. Replace any bars that are used to keep belt from falling off. I am interested in hearing back from you. GOOD LUCK FRIEND.

need to remove the plastic trim where belt retracts to slide up out of way and inspect seat belt retractor drum for debris jamming ratchet or against locking mechanism check belt is not twisted and last look for rust

Sounds like you may have a belt that popped loose. While the belts may look intact, they can sometimes jump out of their groove. Check to make sure the drive belt is routed properly, and that it is tight when the clutch/brake pedal is out (in the drive position). If the belt is routed properly, and the belt is not tight in with the pedal in drive, chances are your tension pulley assembly may have broken, or something else is amiss in the drive pulleys. Also make sure to try all the gears.

There may be a problem with the belt inside the machine.
It has to be taken apart to assess.

check with a elna dealer
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All the clutch does is to remove belt tention by moving an idler pulley, so with the machine stopped, depress the clutch pedal and watch the idler pulley moving away from the belt, and the belt should become slack, if not check all the linkages, if the belt remains tight and the idler has moved away, then replace the belt, it is very important to have the correct belt lenght,

If this is a manual transmission, check the drive belt from the engine back to the transmission. With the clutch pedal up (engaged) there should be no slack in the belt.

If it is a hydrostatic drive, check the drive belt and make sure it is tight. Check the transmission fluid level and make sure it is full. Refer to your owners manual as it will describe the fluid check proceedure.
Good luck, let me know.
Regards,

one thing it could be is the piston freezing up

Have you checked your drive belt(s) for wear or pieces missing. With engine off and clutch/brake pedal up, check the tightness of the belt. You may just need a new belt.