Hi,
Set up the SFTP/FTP account for file import in the external accounts. Check for the file using test activity.
Eg: instance.vars.yourFileNameExist=='Yes'
Then send a notification alert.
If you want to check for continuous 2 days, make use of options in adobe campaign. First day, set the option value as 1 if there is no file. Second day, if no file, set the option value= optionvalue+1.
If file exist, optionvalue=0.
This should work for you.