Bootloader

[Android Central Question]

I changed providers about 6 months ago and turned my old phone off. Today I went to turn on and when i plugged it in a screen came up Tampered Warning Locked.
What happened????
It has been off si ce i changed providers and in the box. When i put it away it was working fine so i can not figure out what happened!!! Please help!!!! Can this be fixed?? It is a htc phone.