Calibre window suddenly won't open

I've been using Calibre for at least 10 years with no problems, but suddenly this week the main window will not open. It is showing on the taskbar, but won't open. I ran the Windows debug command and this is the output.

C:\Users\Nancy>calibre-debug -g
calibre 4.3 [64bit] embedded-python: True is64bit: True
Windows-10-10.0.18362-SP0 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.18362')
Python 2.7.16+
Windows: ('10', '10.0.18362', 'SP0', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 3) && Copy Cover To Device (2, 0, 0) && Obok DeDRM (3, 1, 2)
Turning on automatic hidpi scaling
devicePixelRatio: 1.0
logicalDpi: 120.0 x 120.0
physicalDpi: 95.8113948919 x 95.9160839161
Using calibre Qt style: True

Can anyone tell from this what the problem might be ?
TIA