Are other PDFs still working fine? It's always helpful to have your software updated, so make sure Adobe Reader is on it's most recent version.
Though, if other PDFs work fine, it's probably the one file you downloaded, that is damaged in some way. Try redownloading it and make sure that you got it from a reliable source and not some shady download page. Afterall, PDFs can have malware attached to them, as well.