Keyboard Locks when trying to Get letters with Accents

I have a MacBook and have had no problem for 4 months typing the accented letters I need while learning a new language. But overnight (literally) I can no longer press and hold a letter key to get the pop up with options for which accented letter I want. Now the pop-up appears for a split second and then disappears. As well I cannot type in the desktop that I attempt this in until I click/type in a different desktop. I've looked online but cannot find any solutions. TIA