she withdrew cash, and has spent it, hidden it, or given it away.
I'd think quite likely given it to friends or relations that weren't in the will.
Possibly given to crooks or fraudsters, but how wil you ever know?
As you are executors, you can start by making a register of all the banks and investments you can find, with account number, name and address, latest balance, and writing to them all.
Whether you do the probate application yourself, or pay someone else, this will need to be done, and it will probably be you that has to find out the information even if you pay someone else to put it on a form.
Probate application can be easier than you think, especially if it is below the inheritance tax level (including the unused part of a spuise's allowance).
Beware of old froends and relatives coming out of the woodwork saying "she always promised me..." make a note of it and hand nothing over (yet)
For some reason they were always promised the best jewellery, or the valuable antique. Never the battered old family teapot or the hand-embroidered teatowel.