What did they complain about?
Ramban #2 (citing Ibn Ezra): They had already been one month out of Egypt, had run out of bread and had eaten most of their animals, and to purchase food for a nation as numerous as them would be extremely costly.
And, as the Pasuk goes on to explain, if Hashem wanted to kill them, it would have been preferable to do so whilst they were still in Egypt, whilst they had what to eat - since starvation is a particularly horrific death (See Eichah, 4:9 [Seforno]).
The Rashbam explains that they would have been better off to have remained in Egypt where they would have died a natural death, than to die of starvation in the desert.
Which explains why the Pasuk writes "Vayilonu ... ba'Midbar" Initially, they thought that they would soon arrive at the cities that bordered the desert (Ramban).
Why does the Torah not specify the problem?
Ramban #1 (Refer to 16:2:1:1): Because the problem at this point involved the miracle of the cakes lasting so long, and the Torah does not generally discuss hidden miracles. 1 2: Ramban #2: It actually does (Refer to 16:2:1:3).
Why did they blame Moshe and Aharon for their current problem?
Or ha'Chayim: They blamed Moshe and Aharon for choosing to lead them via the desert.