1999 Nissan Sentra timing belt/chain

Do you mean the starter isn't cranking the engine? If it is, that's "turning over". What is the question? Your engine uses a timing chain. Those don't break real often, but yours does use a number of tensioning devices. If one of those fails, the timing chain can jump a few teeth.

my mistake: the engine does "turn over", so it's not a battery or started problem. it sounds, though, as there's no compression, no firing going on. that's why I asked if the problem could be the timing chain/belt.

This engine isn't an interference engine, so a jumped timing chain won't damage the valves which would lead to no compression. However, if it jumped a few teeth, that would reduce compression.

To verify the chain is not broken, the first thing to look at is if the camshaft is turning, and you may be able to do that by watching the valves through the oil fill hole, as long as there's no baffle in the way. If you can't see in there, you'll have to remove a cover or pull it out of the way to watch the camshaft sprocket while a helper cranks the engine.

If the cam sprocket isn't turning, you know you'll be headed to the timing chain. If it IS turning, things will get a little more involved because you'll have to remove enough covers and things in the way to look at the timing marks on all the sprockets.

You might try a compression test too. If one or more cylinders has 0 pounds of compression, it is likely the camshaft isn't turning at all. If all cylinders are equal but have very low compression, suspect a jumped timing chain that's still turning the camshaft.