Hidden Buttons on our website

Hi! I’ve designed our website using Front Page, but recently, I’ve changed the apparence of the buttons (the unique links to other pages), but since then, some people call me to tell me that they unable to see those buttons and because of that, theycan’t browse the site.
Can someone tell me what happened? I’ve designed the buttons using CorelDRAW 10, exporting them to GIF (Compuserve’s) format. I thought that the main reason was that our visitors had Java disabled on their browser, but I’ve tried with Mozilla and nothing happens, I mean I can’t see the buttons either.
Any suggestions? Thank You!