Aperture library access is corrupted

I (accidentally and stupidly) disconnected an external hard drive from my Macbook Pro 2015 (El Capitan 10.11.4) without first ejecting it after downloading photos from a card into an Aperture 3.6 library. As a result I now can't get back into the Aperture library. I use a 2TB Time Capsule for back-up

Here's what happened:

• After successfully importing images into an Aperture library from a camera SD card, I clicked on the dialogue box to delete the images from the SD card and eject it (my normal routine).
• Then, instead of removing the SD card, I mistakenly disconnected the external hard drive instead.
• When I opened Aperture again (with the eHD reconnected) I got the following message:

  There was an error opening the database for the library “/Volumes/EPCP PROJECT HD/EPCP Image Library.aplibrary”

• I quit Aperture and then reopened it while holding down the Option & Command keys (as instructed).
• This produced a dialogue box with three repair or rebuild options. Clicking on any option produced the result:

  Time Machine is backing up “EPCP Image Library”. EPCP Image Library” cannot be opened until Time Machine has finished.

• Time Machine was not backing up. To be on the safe side, I did then do a back up in Time Machine. I got the same results once more after the back-up was completed. I tried again with Time Machine switched off. Still the same result.

I have tried switching off the laptop as well as the Time Capsule and restarting them. No luck.

I now have three problems:

1. I need to change something in the system that tells Aperture it's OK to repair/rebuild the library
2. I need to recover/repair my library
3. And, vitally, I want to recover the 8-10 images that I was trying to download in the first place.

I need some very simple instructions to achieve this. While I use a computer a lot for regular apps, I have difficulty processing instructions about what's happening in the background - which assume I know what I'm looking at. When I get a dialogue box, particularly with options to choose, unless I already understand what it's about, I really do struggle. I'm a retired person and there is still much of this tech stuff that goes over my head.

• I have read somewhere that the program is fooled by the existence of a file that makes it mistakenly think Time Machine is still actually saving something.
• I have seen reference to .plist and .InProgress files – but I'm quite confused as to what & where these are and what they do - and what I should do with them.
• It does occur to me that if I simply restore an earlier version of the library (from Time Machine) I will probably not recover the latest photos that I had just downloaded.

If anyone can give me some step-by-step instructions, I would be immensely grateful. These images relate to a childhood care history project and mean quite a lot to me (and some other people, too). Also, all the images have detailed metadata stored with them and so I'm keen to preserve that, too.

Sorry this is such a long post.