The display tells you that a startup disk or drive can not be found. The Mac Plus that I owned had an external SCSI connection hard drive (20MB) that I could use as a startup after installing Mac OS 6.0.3. Many of these could be run with limited software from a HD floppy. You probably need to acquire a set of System instllation disks.
Below are some sites that may have a source or some better ideas than what I have given. The Machine looks great so I hope you can get it up and running.
This is a direct link for System 6.0.x from Apple:
Depending upon the amount of RAM installed, System 6.0.3 or 6.0.8 can be used (see the text files). Please note that a Macintosh Plus is limited to working with 2DD/DSDD diskettes in the special Macintosh format 800K. Access to another older (pre-1998) Macintosh computer (with a built-in floppy drive, and StuffIt Expander for the decoding) is needed in order to create floppies from downloaded files (use Disk Copy 4.2).
An Apple User Group could possibly be of help:
In the photo it looks like you have a hard drive; is it connected to the SCSI port? Is it ON when you power-up the Plus? Is it terminated? Does it have a system installed on it?
If it needs to be re-formatted, you will need the System Tools floppy disk.
The Plus WILL start-up from the System Tools floppy, as others in this thread have mentioned.
It seems to me that I have got the same problem with my old MacPlus which has been sitting in the back of the cupboard for yonks. Hard-drive light is green, but the Mac does not seem to recognise it. Everything seems to be connected correctly. I don't have the original software disks anymore and only put it on to see if the originial laser printer that came with it is still working. Any ideas?
It looks like your external hard drive is NOT an Apple-branded hard drive; to format it you will need to patch the utility Apple HD Setup.
The patch is a Godsend. It really works.
Here is the link:
Once you have formatted it, it will be recognized by your Mac. Install a good system on it and you'll be in business.
I'm guessing that these Macs worked originally.
In the first photo I don't see a power light on the external hard drive, is it plugged in and turned on? If yes is there a SCSI terminator on the unused connector on the back of the drive.
In the other photo, could likely be that over time the hard drives heads got stuck. Do you hear the drive actually spinning? If not try very very gently swishing the drive from side to side, this sometimes will unstick the heads/platters. Do not violently shake it! In a really worse case scenario a clean room and a popped hard drive cover and a manual spin up can get it going long enough to recover the data but the drives are typically not reliable at that point.
Buying a used SCSI drive can also be a bit of a crap shoot at this point since it could also be on it's last legs.