The belt has come off of my reebok 1000X elliptical.

This is a tedious process. Leave one of the adjusters where it is and adjust the other side until the belt starts to move to the center. This has to be done with the treadmill moving the belt so that the belt can move into position. Once the belt is in a central position, then you turn the adjustment screws one half of a turn on each one until the belt is tight enough.

I had the same exact problem, and I mean the EXACT same problem. I also changed out the battery with no change. I contacted the generator company thinking it might be that, but they told me it could be the control board. I ordered a control board, changed it out, and now it works like new.

To sum up: Get a new control board.

Front of the elliptical, 4-5 inches up from the ground (directly below the incline adjustment knob).

This says it's a battery cover for the Reebok RL 10.0: http://www.fitnessrepairparts.com/cart/ViewItem/6043840

find that one here.
ask them.

The treadbelt can be installed by removing the front roller, the drive belt, rear roller and screws in the siderail. You do not have to remove both sides, just lift the deck and slide new belt on. To save yourself some agravation, you may cut the old belt off.

Exercise equipment service Treadmills Ellipticals and Bikes Parts Assembly... 877-805-1030

my nordictrack exp1000x doesn't elevate anymore, what can be the problem and how can i fix it?

Check your manual. If you don´t have one, they can get you virtually any Reebok manual.

Ask here for instructions how to change, they can get you any Reebok manual

Check here:
http://manualfox.com/link/reebok.htm

And maybe send them a search request?