A lot of what? If a lot of sth "plural " then you would have to use "were", if you were to refer to lot of sth "singular", then "was" is preferred.
Examples:
There is a lot of things... is commonly used, but is grammatically incorrect because things is plural.
There was a lot of noise in the hall ...OK
There was a lot of junk in this room ...OK
There was a lot of stuff to do ...OK
there was a lot of work to do ...Ok
but were is used for these guys:
There were a lot of things to take care of
There were a lot of cases to handle
There were a lot of questions to answer
so in conclusion since in "There was a lot to do,
or There were a lot to do"
you do not mention lot of what, then both are fine
Again, I am not 100% sure, so don't quote me on that :)