If you have a 2 speed model the first gear in the gearbox (the bigger one) will have a one way bearing in the middle of it. This is a bearing that sits on the gearbox shaft that will spin in one direction but not spin in the other direction. In cold or dirty conditions it is possible for one way bearings to fail as if the grease that is on the bearing gets to cold or dirty it solidifies and causes the bearing to slip. If this happens the first thing to try is to warm the gearbox up for 5 - 10 minutes using a hairdryer which will hopefully solve the problem. If it does not you will need to take the gearbox out and take the first gear off to reveal the one way bearing and shaft. Using some wd40 or similar you need to give both the shaft and the middle of the one way bearing a really good clean so there is no grease left on it at all and then re-grease it using one way bearing grease. Taking the gearbox out is easy but takes a little time. Undo the four screws on the bottom of the chassis that hold the gearbox on and then undo the screws that hold the upper chassis on. You will not be able to take the upper chassis off as the servo wire will still be connected but you will be able to move the upper chassis so that you can then get the gearbox in and out ok. Getting the drive shafts back in the drive cups is a little fiddley but if you use some pliers or something to hold them it's not to difficult. If you clean the bearing and it still slips just let us know as it may need replacing.
Other possibilities why a nitro model may not move are:
1. The brake pads are to tight or have got wet and have expanded causing the brake to be constantly engaged. If this happens you need to loosen the brake pad screws.
2. One of the grub screws in one of the drive cups has come out or is loose. If this happens the drive cup will rotate but it will not turn the shaft. You should go round every drive cup and make sure all the grub screws are tight.
3. Something inside the differentials may have broken or come loose. You would need to open the differentials and check all the cogs inside are ok. If any cogs are worn it would be better to replace them but you can get a bit more life out of them by padding them with washers to push the cogs closer together and improve the mesh.