Is it still usable? This is a DVD+R if it matters.
the filesystem is not finalized and thus is corrupt in general, unless your burning software is so intelligent that it will finish off what it is doing (i do not know too many burning softwares so do not know, but presumably this could be done). so the short answer is you will have something not usable, and cannot be used for reburning.
you can however open a cd/dvd for multiple session write, in which case you also need to wait for a session to finish.