Hi mate,
When the bonnet won't close, is the bonnet release handle in the footwell loose (ie no resistance when pulled). If so it means one of two things, your cable is knackered and may be about to fail. See guides section on what to look for on the cable. Or the spring locking pin is sticking. They sometimes stick when they get clogged up with grit/grime/rust and the grease almost exacerbates it. You can try stripping it down and giving it a clean with degreaser or wd40 and see if it loosens it up.
If the catch/ cable is springing back as it should, then as old hammer says it might be the bonnet is not aligned correctly, although unless you've had a minor bump or new bonnet etc, if it's been working correctly up untill recently I'd suspect the cable or catch. There are two little rubber buffers on the front slam panel either side by the headlight, you can try screwing these in by say 1/4 of a turn at a time to let the bonnet sit a bit lower into the catch, but aesthetically you should line it up so it sits flush with the top of the wings.
Alan
Edit atom beat me!