My ip address is not seen by other computers in my work group

Mac Mini OS X.10.2, 2.3 GHz i5, 8 GB 1333 DDR3, 500 gb hd

I can not ping my Mac from other computers in my work group (both PC and Mac). I've been sharing a folder for a few years in a small work group and all of a sudden that folder can no longer be seen or mounted mounted on other computers in my workgroup. I am connected to the network and internet and I can ping other computers in my group. They can't ping me (get no response when pinging my ip address) and therefore are unable to see me to mount and share my folder.

I've double checked all my sharing setting, permissions and find everthing as it should be, but others are unable to mount my folder.

Thanks in advance for your input.