well this is difficult. why not invite them round and show them the problem and see if they will put drains in between your land and theirs. if not perhaps you might consider a land drainage system for your own garden. Was this work down this autumn so its the first time the problem has arisen or have you had a lot more rain then normal.

try not to fall out with the neighbours as this can make you more unhappy and in the long term may make you ill.

welcome to GoY too :o)

Could you give us more info, lay of the land etc, size of garden, you could do like seaburngirl says, put in a drainage system and divert it to a large soak away, we had similar situation over in france, our house is in a deep valley and a stream up in the village was diverted by the commune workers and the stream found a new path into our garden and as we were in process of installing a fosse it became a nightmare, we dug a large deep trench filled with gravel and this now works well and takes the water away frm the house, you your could do something similar using special drainage pipes.

Before railing on your neighbor, keep in mind that the two events aren't necessarily related. I live "above grade" from my neighbor, yet his garden never floods. The ground can handle the excess rainfall for a good long while before any signs of floods, even if his property was paved over. Clay soil very prone to pooling. If you are on clay, that may be the issue.

A difficult situation with drainage had problems myself and this year decided to put in land drainage cost me about £200 but so far had no more problems useful to watch videos on youtube.

Since the neighbor changed the grade of their property which caused increased water runoff into your property this is not a natural occurrence and is subject to legal recourse. Make the party aware of this situation and request that they make a satisfactory remedy. If they used a landscaper have them give you the name of same. If they do nothing or are not successful go to a lawyer. Do not make any changes to your property to correct their error unless they are willing to pay for the consult and correction. One final thing don't worry about making your neighbor angry, perhaps they are counting on you to grin and bare it.