'sup Georgia_boi. Your question is not a simple one. Phasing can be caused by EQ'ing two different instruments on the same frequency in a mix. What I'm about to explain applies to any sound or instrument/s you may have used, but for example here's a common error...On many occasions, people tend to EQ two or more basses/bass instruments on the same frequency and when those basses clash (since each has it's own unique sonic properties) phasing occurs. The instruments may be either 180Degrees out of phase (that's when they "disappear" because they cancel each other), 90Degrees out of phase (they'll become louder in level...like 1.414 times louder than either sound or somethin' like that), and lastly, when they are neither 90Degrees or 180Degrees outta phase, that's when comb filtering occurs (random degrees outta phase).
But seems like you have the 180Degrees phasing problem (since you described the sounds as disappearing), so I advise you double check your EQ'ing on the different sounds/instruments you EQ'd.
GOODLUCK!