How do I recover my Ubuntu partition?

[s_s_shahin at fastmail.fm]

I divided my hard drive into 3 partitions, 1 Ubuntu 16.04, 2 Windoz and
was booting Ubuntu day in and day out no problem until I wanted to try
one of the Windoz.

Now Ubuntu will not boot and the "disk cannot be found".

I know it is there but I need to recover it.

TIA

Suleiman Shahin

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