My friend lied to me what shall i do?

i hav this really good friend but she lied to me, she said she was grounded so she couldnt go into town wiv me and we had planned it for weeks !! then i found out that the next day that she had gone into town wiv a different friend on that day that we were ment to being into town . i found out when i was walkig in town and i saw them and they ran what do i do, do i take it seriously??