The Dungeon Master's Guide (3.5) p.296 and Rules Compendium, p.49 describe the process of level loss, but say nothing about regaining levels. Hence, there are no restrictions on what new levels you can take.
According to the D&D 3.5 FAQ, regarding a character who suffers level loss after being killed and brought back with a spell like raise dead:
The character is under no obligation to make the same selection(s) when he next gains a level with XP.
The only restriction is that if you use restoration to quickly restore levels lost to level drain, unlike gaining new levels normally by acquiring XP, that spell only restores the same class level you lost:
A character who regains a lost level from restoration or a similar effect must regain the same class level (and other abilities) that he lost. In other words, you can't use level drain and restoration to rebuild your character.