Your power supply controller may have died. Remove the jumpers as Rob instructed and measure the voltage between ground and each of the pins of the headers for the 3.3v and 5v rails. Note that one pin of each two-way header is connected to the power supply and should measure the correct supply voltage, and the other pin is connected to the circuitry and should measure nothing.

Also, remove the power and measure continuity between both pins of all the headers and ground. It should be a fairly high resistance, hundreds of ohms up to several K.

If this is all OK, fit the 3V jumper and power it up again. Measure the 3.3V power supply on the jumper if it has an open top (most do). If it's OK, measure the voltage between the pad near the processor labelled VCORE and ground. This should be 1.8V. Be careful not to slip with the probe on this pad and short it to anything. If this is not correct but 3.3V is, the vcore regulator has failed.

pca