The good news is that your problem could only be one of a few things; the bad news is that the faults involved could have several causes, and you’ll need to track the problem down before you can fix it. None of these are particularly difficult to find, and you don’t even necessarily need a multimeter to find them. A few basic electrical and hand tools, a few pieces of wire and some free time are all you’ll need to troubleshoot and find the source of your cooling failure.
Check your fan belt and fan blades if you have a mechanical cooling fan without a clutch (it only moves with the pulley). If you have a belt problem, then you’ll know it right away because the water pump won’t work. If the fan blades have gotten bent or tweaked, they might not pull the requisite amount of air.