You do have a multimeter, don't you? If not, first step is to buy one. A cheapy will do, you can buy a pretty decent DMM for less than fifty bucks these days.
OK, go to the where power comes into the panel. Check hot to neutral on each leg. Check it both on the actual conductor coming in as well as the lug where it's attached. You may find it's good on input cable but bad on the lug; that's a common problem if the screw wasn't properly torqued down and started arcing. At any rate, work forward or backwards until you find where you're losing power.
Tip: Check the voltages under load, meaning the switches and breakers are turned on, and you have something turned on for each leg. Otherwise the voltage may look good if though the connection is bad.