Guitar - General/Floyd rose bridges
My guitar can't go to E standard tuning because my bridge tilts upwards, I went to Guitar Center to see what was wrong and they said it was because I took off all the string at once, is this true?
Most likely, yes.
You should never remove all of the strings at once, regardless of bridge type, especially on a Floyd Rose system.
Necks are designed to allow for a certain amount of leeway in pressure, or variation in torque, over a limited duration while changing and tuning strings.
Similarly, bridges of complex design, such as the Floyd Rose models, require adhering to a meticulous process when changing strings.
Changing one string at a time is ideal for both the neck and the bridge.
The reason I said most likely yes is because your guitar might have other issues that need addressed, of which I am unaware as I am unable to physically see and test the condition of the instrument.
Removing all of the strings definitely introduces the issue of an off-tilt bridge.
I encourage you to read or acquire then read the manufacturer's manual that applies to that exact model bridge. If you are unable to level the bridge, ask a local instrument repair technician to level if for you.
Best of luck!