Assemble, first, all your family members. First, it alone will be difficult to search bills. Secondly, you need to make sure that your roommates could not be involved in the loss of money. So, take a piece of paper and a pen. Then make a list of all the possible places in the house where you can lie bills.
Get familiar with the chronology of the loss of money. Remember and record the places in the house where you could hide (put) money. It would be nice to write the exact date of these transactions. Remember, where you could take that money and spend it. In many cases, people simply in the inertia take pending bills and sent them on an urgent matter. Maybe in your case the same thing happened.
Recalculate several times all their savings and ask others to do the same. It all depends on the honesty of your roommates. In the process of this review, there is a great likelihood to figure out whether you lost money or did someone take. If that doesn't work, start organized a search around the house.
Check all shelves, folders, Newspapers, magazines, paper and books. This is the most common place where I could drop the bills. Often they can be between the pages of books are on the coffee table. Also, the money can be in documents: a passport, a ticket, a pass, etc.
Almost search the whole house, looking even in places where they might not even be in your opinion. Look under the carpets, flooring, beds, in corners and in pockets of clothing. Search for until, until you find it. If you could not do, then there are 2 options: either you are not correctly counted their savings, or they took the one who could penetrate into your home space.