This shouldn't happen. When you run a command in the background bash usually just prints the job number and pid (see below) and then drops back to a command prompt.
The only reason I know of that leads bash to leave you with a new line in this situation is if your command doesn't actually exist, as it will somehow give the error message for that after printing PS1:
$ nonexistant_command &
[1] 13856
$
nonexistant_command: command not found
<empty line with blinking cursor here>
Whatever the cause may be, pressing enter while on this empty line should print a new line with your usual prompt (and, possibly, the message that the job you just spawned finished; see below).
The additional output you see is bash telling you about current background jobs:
First, it tells you the current job number of any newly created background process together with its pid, like so:
$ some_command &
[1] 1234
This means that the command some_command
is now being run with the pid 1234 as job 1.
Once a background job finishes, bash will notify you of this the next time it prints a command prompt (whether it be because you spawned another background process, ran a command in the foreground or simply pressed enter with an empty command line) by printing a message like this:
[1] Done some_command
This means that the job with the number 1 which ran some_command
has now finished.
To find out what background jobs bash is currently tracking, you can use the jobs
builtin like so:
$ jobs
[1] Running some_command &
[2] Running some_other_command &
To bring a job to the foreground, you can use fg
:
$ fg 2
some_other_command
For more info on this, try help jobs
<return>
again? do you get a prompt?