Timing belts, or chains, are the part of your engine that keeps the camshaft, distributor, crankshaft and pistons in sync. When the timing belt breaks or is damaged it can keep the engine from starting up. Most timing belts will last about 60,000 miles (96,561 kilometers) but if your engine has been sitting for a long time it might be a good idea to inspect it.
Many cars use reinforced rubber belts, but older cars and many newer luxury cars still use metal chains. In either case, a broken timing belt (or chain) will inhibit your engine from starting.
Although it may take only a few minutes to actually slip the belt on, getting to the belt might not be all that easy. It's likely you'll have to take off multiple hoses, move electrical wiring and remove other engine components to gain access to it. Once you're able to access the belt you'll have to make sure to align the crankshaft and camshaft markings and ensure the number one piston is at top dead center (TDC). If you don't get this exactly right you can throw the entire engine off its timing and probably cause some serious internal damage when all of those metal parts start crashing into each other.
Needless to say, changing the timing belt isn't for everyone. It requires a high level of skill, and in some cases, it might even require taking off an engine mount to gain full access while suspending part of the engine as you work on it. If your timing belt or chain is old, stretched, worn or broken though, you'll need to replace it to get your engine started. Consider taking it to a mechanic if you don't have the tools and skill required to replace it.