The aboleth is now stuck in the Plane of Water until they find a way back.
Nothing in the aboleth monster description says that they automatically return to the Material Plane when their body is reconstituted. Their statblock has no planar teleportation ability either, so unless the DM asserts otherwise, the aboleth has to find their own way back if they wish.
An aboleth can still use normal methods to travel back to the Material Plane. They can find a planar portal, acquire a minion spellcaster who can cast plane shift for them, and so on. How this travel works depends entirely on the portal or spell used. It might take years or decades to find a portal, or the genius aboleth may have even prepared one ahead of time. This leaves a lot up to the DM.
The ability of the aboleth to reconstitute is new to D&D 5th edition, so we cannot consult earlier editions' sources for how this ability works. The 5e sources do not give much in the way of additional clarity here.
We know from its Monster Manual entry that some aboleths actually live in the Plane of Water, meaning that a reconstituted aboleth has the option to stay:
Aboleths dwell in watery environments, including ocean abysses, deep lakes, and the Elemental Plane of Water.
Tales from the Yawning Portal, p.134, has an aboleth who is trapped in a pool, implying that they cannot simply teleport or plane shift to escape.