I'm amazed no one has tried this:
Sounds very much like you just put them in with the wrong order.
Make sure that you've put them into the same slots as they was in when you took them out, if it's only 2 sticks, then all you have to do it to just switch the order.
If uncertain of which slots they was in: If your mothermoard is not too old it should have 2 slots in one color and 2 slots in another color, try first to put them into into 2 slots with the same coloring, if that does not work try putting them in slots with different coloring.
If you got more than 2 sticks, and all look the same (both on the sticker on it and the design, brand etc..), just make sure to put them in the order explained in the note below in the 3 slots closest to the CPU.
If more than 2 sticks and at least 1 of them are the slightest different from the other, put them in with the order described in the note below, but but the odd one(s) in the differently colored slots.
NOTE: The slots are always numbered 1, 2, 3, 4 (and so on). The slot closest to the CPU should always be #1. If your motherboard has 2 slots in a color and 2 slots in another color (and 2 in a third color if 6 slots) the other slot with the same color should be #2. The slot closest to the CPU in the other color is #3 and the remaining slot would be #4 (you get the idea). If all slots are in the same coloring, it should be in order 1-4/6 from the one closest to the CPU to the one furthest away.
The order is that if all sticks does not have the same amount of memory per RAM you should put them in the 2 slots closest to the CPU with the same color (if more than 2 sticks you just put the remaining stick(s) in the remaining slots in the mentioned numbering order.
Main point is that you put them in with the numbering order with the rules mentioned above.
Short version: If they look the same and have same amount of memory per stick, put them in with the numbering order mentioned in the note above.
If they have different amount of memory per stick put the ones with most memory per stick in the slot #1 (once again, using the numbering order mentioned in the note) and in decending order. If this does not work, try reversing the order and putting the one with least RAM in first instead and ascending order...
If 2 or more sticks are not alike but have same amount of memory, you put the one most alike the other sticks (if any) closest to them (see example)
For examples:
3 sticks, all with 1GB per stick and same model etc.
Just put them in slot 1, 2 and 3, no wierdness there.
3 sticks with 1 having 2GB on the stick and the other 2 having 1GB per stick, same model all of them:
Just put the 2GB stick in slot 1 and the other 2 sticks in slot 2 and 3.
If this does not work, put just the 1GB sticks in slot 1 and 2 and the 2GB stick in slot 3
(This example applies even if the 2GB stick or both 1GB sticks is from a different brand or model)
3 sticks with 1 having 2GB on it and the other 2 having 1GB per stick, one 1GB stick from a different brand or model:
Put the 2GB stick and the 1GB stick from the same brand of model in slot 1 and 2, put the remaining 1GB stick in slot 3
If this does not work, just switch the position of the 2GB and the odd 1GB stick.