If you are definite that you are missing a unit that you previously owned, then the most likely reason is that you might have accidentally sold it for zel or fused it to another unit.
If the unit is really important, you can request for it to be restored, but be warned that there are only a limited amount of restorations allowed per account. Once your account has reached this limit, no further restoration requests will be entertained.
A general rule is that only special event units or units from the Rare Summon Gate will be restored. Common units such as Minotaurs, Unicorns and such will also not be restored because they are easy to acquire from quests and are commonly fused away by players for increasing Brave Burst or for experience. Metal and Jewel units will also not be restored for accidental fusing or selling and the same applies to Burst Frogs.
For unique units which can be only be acquired once, such as the mock units (Karl, Grahdens, Maxwell, etc.) they are only entitled to be restored once. So be careful once a specific unique unit has been restored, the Brave Frontier team will no longer entertain if ever it occurs again.
Restored units will always be Lv. 1 when returned. Restored units may be at the evolution level of when they were summoned or at the latest evolution level they were sold at. Aside from this, restored units will always be Oracle in type, unless it is a unique or mock unit which is specifically a Lord type by design.
To make sure that your favorite units are not accidentally fused or sold, be sure to make them join a squad or use the Favorite function. If you want to know more about how to avoid accidentally selling or fusing your unit you can check a quick guide here.
Disclaimer: The Game Support team will only entertain this kind of concern within a limited amount of time or days, which is seventy-two (72) hours or three (3) days starting from the time or date of occurrence. The reason for this is if a lost unit is important to you it is imperative that you report it to us immediately since it is the players' responsibility to take care of their respective accounts. We are reminding everyone to be very careful and be aware of this provision.
If you are sleepy, drunk or intoxicated, please refrain from fusing and selling as much as possible. ^_^ - gumi Asia Team