It's probably pretty manky. Depending on the chlorination levels of your water, it will keep for a couple of months. You'll probably want to invest in some cleaning supplies for the bladder and tube. Many use denture tablets and report reasonable results. If you have silver impregnated tube and bladder, that really helps too.
For the tube, a swab of some sort is handy. Camelback sells a kit that simplifies things.
Yes it will go bad. I don't know that particular container but I'm surprised to hear that water could hold for 2 months. I'm using a filtering water jug here and if we don't clean it and keep fresh tap water in it for about a week we can see green stuff starting to grow inside. Might not kill you but it's not a pretty sight.