It is likely that your browser has timed out if you have had this particular Send window open for a long time where you have been scoring out individual email addresses to exclude from your email campaign.
If you wish to exclude lots of contacts from within tag groups or the whole database then really you should create a new tag group and apply this new tag to just those contacts you wish to include in your email campaign.
Please click here to see how to create a new tag.
Please click here to see how to apply your new tag to your contacts in bulk.
That way you will be able to select your new tag group and send out your email campaign to just this tag group - saving you lots of time as you will not need to score out lots of individual email addresses.
To view a video about sending out an email campaign please click here.