Microwave not working, convection OK

I would suspect the magnetron. This is what produces the microwave radiation and could be a bit pricy. NOT a fix it your self item. Microwave circuitry can be complex and dangerous. Best to take it in to a repair shop. Or, depending on age might be time for a new one. magnetrons usually fail very soon (warranty) or will last a long time if not abused (micro wave run with nothing (or very small amounts) in it. Like reving an engine with no load.

It could also be a fuse which is again probably for the shop.

The one most common fault in any microwave oven (aside from the fuse) is a bad door interlock switch. There are two of them, and they are relatively easy to replace, and can usually be purchased at any leading electronics parts supply. Also check the wire connections to those switches; often they show signs of overheating, and simply need trimming and reconnecting with a new connector.
Repairs beyond that are out of the realm of the DIYer, as they involve replacing capacitors, HV diodes, magnetrons (unlikely), etc.
I'm assuming that the fuse is good, as you say the unit does appear to have power.

Brand and model?

Dead magnetron. It happens sometimes gracefully (slowly as in stuff takes longer to cook) or all at once (worked this morning just fine). Might be more cost effective to replace the oven over repair but doesn't hurt to check.

I would search for a new microwave to replace your unit. Just the labor and parts cost to repair your older unit will probably buy a new microwave with more features.

I would check the ceramic fuse first, they are cheap and easy to replace.