Hi, my camera was working fine and then in between taking pictures all of a sudden it said "memory card error" and now I can't see some of the pictures I took. Please help!

First don't not take any more photos on the faulty memory card!Do you have a second memory card? if so does the second memory card work?. If the second card works you could try some file recovery software. I use one that came with my, but you could try http://www.z-a-recovery.com/digital-image-recovery.htm ( I have never used it but does have a free image recovery option).
I have managed in the past to recover quite old files from memory cards, the important thing is to not add any more data to the card as this can overwrite what is already there. Unfortunately there are no guarantees for data recovery. Hope it all works out

If the camera displays Memory card error, No Memory Card, or takes an abnormally long time to write an image to the memory card, when a memory card is inserted or any of the above error messages, make sure that the card is inserted properly. For SD card, make sure that the card lock switch is in the correct position and not stuck in-between. If this is the case, you may wish to Format the memory card in the camera using the Setup Menu option [Format] as shown below. Please be aware that formatting the memory card will remove any images currently stored on the card along with any other data. This can also be caused when images shot with another camera are stored on the memory card.
If after formatting the card in the camera the error persists, try another memory card. If this occurs with only the one card it is likely that the memory card is defective, if the error persists over more than one card the camera may require service. If the card appears to be defective you may wish to contact the memory card manufacturer.