I don't use Hotmail but have just used the search term 'hotmail emails blank' to see if I could find anything out. It seems you're not alone in experiencing the problem.
What browser are you using? Have you tried the usual emptying of cache just to be sure it's nothing that simple? Also, have you installed anything recently such as Firefox extensions or have you altered any firewall or security settings? How about sending some e-mails in
plain text to see what happens. Perhaps you are using HTML but your friends don't have HTML enabled in their e-mail program. If nothing has changed at your end, then perhaps it's glitch at the Hotmail end that may be only temporary.
Have a scan with your security programs just to be sure nothing is wrong. Probably not but if you take the 'try everything no matter how unlikely' approach, it sometimes pays off.