What happens to air mail letters when postage is not enough or the stamp is missing?

I posted a large letter today at post office and beacuse i had headphones inside which i wanted to send to malta, the top right section of envelope where the stamp is sticked was a bit rough/wrinkly so the postage was not able to stick properly but with more force and saliva lol, i was able to stick it (i think) but if it came off or went missing, what happens?