Disappearing Gem

Has this ever happened to anyone in the Ruby world? I was working on my
home computer for several hours doing normal debugging using ruby
logging. Then decided to take a look at log4r. I rewrote some of my
software to use log4r, and made several trial executions to find the
errors. After the third (or fourth) such execution during which I
believe I found and fixed some problems, I started getting the message:
“…cannot load log4r.”

In the normal course of events, I would have expected to get such a
message at the first crack at execution, even before syntax errors which
would be buried further down. The message is most unhelpful since ‘gem
list’ clearly shows the gem is there (could it be corrupted?)

The full text of the message follows:

C:/Ruby200-p481-x64/ruby-2.0.0-p481-x64-mingw32/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in
require': cannot load such file -- log4r (LoadError) from C:/Ruby200-p481-x64/ruby-2.0.0-p481-x64-mingw32/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:inrequire’
from GameWinTk.rb:3:in `’

Any help or suggestions would be greatly appreciated, thank you.