pondpaul, what finnipper says is right in some respects, not in others. (Sorry finnipper)
The problem you have is that the local water table is too high.
Its not just because the water came from under the fence, water from everywhere sinks into the ground (where it can) and in so doing raises the water table.
The only real solution is do dig a hole about 15cm lower than the pond is deep, put a paving slab at the bottom of the hole, put a wide drainage pipe onto the slab, make it so that it is about 4cm below ground surface level.
Backfill around the pipe with gravel. Put a sump pump with float switch in the pipe. (So it stands on the paving slab) Run a pipe from the sump pump to waste.
Switch the pump on and watch, make sure the float switch can move easily, put another paving slab on top of the pipe. Done.
If you just dig a hole and put in a sump pump as the water is pumped out so the walls will collapse and the soil will be pumped away making the hole bigger, and bigger. the pipe and paving slabs prevent this.
Raising the level of the pond sounds to be a good idea, the down side is it can not be guaranteed to work, not to mention you will have to buy a new liner.
The reason is hydraulics. If the water table around your pond is rising and you build a wall around the liner you are in effect building a restriction, so by the laws of hydraulics you are making things worse.
To be honest, all you can realistically do now is prepare for the next time.
Good quality costs. Bear this in mind before you start.
Sorry if my reply is NOT want you want to hear, but what I have said is true.
We can only go by what you type.
A "thank you" costs nothing, but goes a long way.