When the notes client crashes there are a list of processes that need to be shut down before it can restart, this usually results in a restart machine or desktop error message.

nadminp.exe
naldaemn.exe
namgr.exe
napdaemn.exe
nchronos.exe
ncollect.exe
ndyncfg.exe
nhldaemn.exe
nimapcl.exe
nlnotes.exe
nlogasio.exe
nminder.exe
nnotesmm.exe
nnsadmin.exe
npop3.exe
nupdall.exe
nupdate.exe
nweb.exe
nwrdaemn.exe

Do a CTRL - ALT - Delete and click on processes then end them and restart notes,

or just restart the machine.

Alternatively you can use utilities to shut these down:

http://www.notes.net/sandbox.nsf/ecc552f1ab6e46e4852568a90055c4cd/bf0390d5cb512596852568c8005107d4?OpenDocument&Highlight=0,zapnotes

KillNotes.exe is another I have heard of and also

you can use Notes Medic available from:

http://www.cassetica.com