I can't play my svr (Exception ticking world java.lang.OutOfMemoryError: Java heap space)

This is my first post, sorry if I do mistakes.

Whenever I try entering, I can only play 5 seconds before the error pops-up: 

Exception ticking world

java.lang.OutOfMemoryError: Java heap space.

 

Yesterday I could play minecraft perfectly, and today when I tried to log in, this error appeared.. if someone could help me I would be really thankful.