contact customer support?
depends on the message you got
theres a bug going around saying anti cheat blocked you, but thatll resolve by just restarting the game, just a false flag thingy due to connection issues or something idk

also if you used any third party software, like cheat engine or qwerty rebind or something, anything, even if you closed it before starting up SE5, theres a chance itll still be detected
so if thats the case, restart your pc and play the game without ever opening any such programs
ANY third party software, not necessarily one designed to cheat, like qwerty rebind
oh...some items like special mouses or keyboards are also not allowed
like those mouses...mice? where you auto dodge when aiming or can repeat shoot while holding down the button....all that kinda stuff

and be sure they wont auto start in the background when you do start up your pc

maybe reinstall EAC?
verify game files

Mine's doing this often now too :(