We had a new serpentine belt replaced a couple of months ago and it just came off. How can I put it back on? Also the lug nuts seem to be stripped.

Click on the following free direct Link. It has the Serpentine Belt Diagram and Tensioner location for your 2000 Saturn SL.

http://www.2carpros.com/car_repair_information/year/2000/make/saturn/model/sl/2000_saturn_sl_drive_belt_routing_diagram.htm

Before we get to putting it the Belt back on...
Look at the alignment of the Pulleys. Most times the Tensioner or Idle Pulley are "canted" - meaning pitched or leaning forward, which causes the belt to "run off" the pulley - throwing the belt.
With the Belt off: check side-to side play for each one of the Pulleys by hand, especially for the Tensioner and Idle Pulleys. There should not be any side-to side play. If there is any other Pulleys that are canted - check if they are properly tightened. A new belt should not have come off on it's own without some other problem.

GENERAL/Standard Rules to Removal the Serpentine Belt with a Tensioner:
A. Find the Tensioner(s). (See Diagrams on Link)
B. Using a Serpentine Belt Tensioner Tool, Or Wrench OR Ratchet, Rotate/Move the Tensioner USING THE AVAILABLE NUT (on the Pulley or Arm Base) OR the OPENING (3/8ths or 1/2 inch square opening) rotating the Tensioner Pulley/Arm - moving it away from contact with the belt and towards the area where the belt is not in contact with the Tensioner Pulley. The Tensioner is spring loaded, and is hard to rotate/move.
C. Remove the loosened belt off one of the other more easily accessed Pulleys.
D. DO NOT quickly release the Tensioner, but gently allow the Tensioner to rest in its closed position.

Standard Rules for Installing the Serpentine Belt with Tensioners:
a. Pick a Pulley that is most easily accessed. This will usually be on top. This will be the last Pulley that the belt will go on.
b. Using the Diagram: Install the new Serpentine Belt on the remainder of the Pulleys....over, under, left right.
c. Using a Serpentine Belt Tensioner Tool or Wrench or Ratchet Tool: Rotate/Move the Tensioner Pulley/Arm "away" from Belt contact area on the Tensioner. This spring is pretty hard and with a new belt, it will be even harder to install. Rotate/Move this to as-close to the maximum allowed inorder to have enough slack in the belt to get it up and over the last pulley.
d. Using your other hand - Pull the Belt up and over the Last remaining Pulley.
e. Before releasing the pressure on the Tensioner, visually inspect the remainder Pulleys and the Belts' Positioning on them.

Regarding the Strip Lug -Studs.
Is this on the wheels with Rotors (Disc) or Drums? If the tire was not properly mounted, or the Lug Nuts were not properly Torqued; this will cause the Lug Studs (bolts) to be stripped.
With the vehicle raised and properly supported by jack stands:
For Rotors:
Remove the Tire, Brakes (Calipers/Pads) and Rotor, exposing the Wheel Hub (Bearing) Assembly with the Studs fully exposed. Rotate the Wheel Hub to a location where the Lug Studs can be clearly removed FROM THE REAR OF THE WHEEL HUB ASSEMBLY.
Using a impact hammer or mini sledge hammer: Hammer the Lug Studs back thru the Wheel Hub Assembly.
Remove the loosened Lug Studs.

For Drums:
Raise, support, remove the Tires.
Remove the Drums off the Wheel Assembly.
Using a impact hammer or mini sledge hammer: Hammer the Lugs back into the drum, removing from inside the Drum

Purchase the new, correct size and length Lug Studs and new Lug Nuts. If the Studs were stripped, most like the Nuts are too.
To reinstall the Lug Studs: Purchase an additional two (2) Lug Nut that fits that Lug Stud. Also purchase several Large Heavy Duty Washers that can fit on the the Lug Studs.

Place the Lug Stud back into the Wheel Assembly from the Rear (Rotors) or from inside (Drums). Use High Temp Grease for the area of the Lug Stud Splines that have to be "drawn" into the Wheel Assembly (with Rotors) or into the Drum.
After the Lug Studs are "IN" as far as they will go: on the outside place at least two (2) of the Washers on Lug Studs. Use a Lug Nut and tighten as far as possible down-on the Lug Stud as possible.
Next: Using an Impact Wrench tighten that Lug Nut down/on, drawing the Lug Stud into the Wheel Assembly/Drum. Check the rear of/inside the Wheel Hub Assembly/Drum. The Heads of the Lug Studs should be firmly against the Wheel Assembly/Drum.
If you do not have a Impact Wrench - use at least a 24-in Breaker Bar and Tighten the Lug Nut on, drawing the Lug Stud into the Wheel Hub Assembly/Drum. This takes longer and alot more work, compared to 10-seconds using the Impact Wrench.
Repeat as necessary.

Let me know if this helped, or if you have additional information or questions. Feel Free to contact me at FixYa.Com!