If you opened the recorder application, then started a new recording AFTER your phone rebooted itself you've probably lost the file..

However,

If you've not made a new recording since I think you could try either of these two things

1) Use a PC and mass storage mode:

Plug you phone in to a PC, connect in mass storage mode. Use explorer (or finder) to open the mass storage on your phone. Look for a folder called ".recorder" Inside it you should find a file called untitled. Move this file out into the parent folder where you saw the .recorder folder)

2) Using the phone

Open an x terminal and enter these two commands