Errors while sending E-Newsletter

I've sent several e-newsletters successfully to my list, and suddenly they're not working anymore. My e-newsletters are saving, but this error happens when I attempt to send them out - so they don't get sent. The errors are listed below - I've searched the forums a bit, but I'm not seeing much in terms of solutions... thanks!

Warning: Missing argument 2 for wpdb::prepare(), called in /home/tmeitner/ on line 588 and defined in /home/tmeitner/ on line 990

Warning: Cannot modify header information - headers already sent by (output started at /home/tmeitner/ in /home/tmeitner/ on line 876