If you're literally sitting at the bootloader screen, the volume buttons will scroll between boot options and the power button will select. Simply use the volume buttons until it has "Start" selected and then press the power button to select it. This will boot your device normally.
If for some reason it's stuck at the bootloader and won't accept input, removing the battery should be fine provided that you aren't in the middle of executing a fastboot command of some kind. Note that if you're getting a response of
<waiting for device> from fastboot that it is not doing anything on the phone's side. That means that it cannot communicate with the phone, and as such the bootloader itself is basically idle at that point.
Since you noted in a comment that you were concerned about the bootloader erasing things: the bootloader process in and of itself doesn't do this. The purpose of the bootloader is to allow you to boot into various different modes of the phone (such as recovery and a normal boot). In the PC world a bootloader is often used to dual boot two different operating systems, with the bootloader menu being what allows a user to select which OS they want. The only time that your data will be erased is when you execute the
fastboot oem unlock or
fastboot oem lock commands or if you explicitly erase a partition in some other way (
fastboot erase, for example). Simply booting into your bootloader does not affect your data in any way.