Okay. You're trying to enter the elevator room. To enter the elevator, it must first be activated (1:42). The time the elevator is active is limited. Once that time has passed, you have to reactivate the elevator (1:42) to re-enter. That is, you have to return to the room with the 'LIFT' arrow and make it point to 'ON' again. It's not the only element that's active for a limited time. The same applies to any arrow you have to jump on (like at 1:42).