There are two distinct types of shortcut keys: those assigned via
customization (as per Idaho's post) and those which invoke menu commands
(for example, the Alt+I that you have lost).
The ones that invoke menu commands are called Accelerators and exactly which
ones are in effect, and exactly what they do, depends on the menus you have
displayed. Accelerators are always Alt+(some key), the (some key) being the
underlined letter in the menu item. They can be overridden by
customization, but it is good behaviour, though not enforced, nor
enforceable, to not use Alt+(some key) for that.
If you have lost your Alt+I accelerator for the Insert Menu there are three
things that could have caused it:
1. You no longer have Insert available as an option on the menu. If this is
the case you can reinstate it via customization, in a few different ways,
the easiest being simply to reset the Menu Bar. Go to Tools > Customize >
Toolbars tab, scroll down and select Menu Bar and then press Reset. Do be
aware that this will also reset any other customizations that you have done
to the menu.
2. "I" is no longer set as the accelerator for the Insert menu (it is not
underlined). This is unlikely to have happened by accident but can be fixed
as per (1) above of you have no other customizations.
3. You have Alt+I set as a shortcut for some other command. This also seems
unlikely as you don't say something else is happening when you press Alt+I.
To check, press Ctrl+Alt+Num Plus: the cursor will turn to a cloverleaf
shape; now press Alt+I. If you have a customization set, the customize
keyboard dialog will show and you can then remove the assignment. If you
have a menu accelerator set, the menu will pop up; otherwise nothing will
happen - and pressing ESC should clear the cloverleaf cursor mode.
There is one other possibility - that you have multiple visible menu and/or
toolbar commands that could lay claim to the Alt+I accelerator. In this case
Insert (on the menu) will simply be highlighted rather than opened up. If
this is the case, repeatedly pressing Alt+I will scroll you through them and
you can identify - and do something about - the pretender - either close the
toolbar, or remove it from the toolbar, or unasign the accelerator from it.
To do this last, again select Tools > Customize, and while the dialog is
open, right click on the offending toolbar icon - there you wil be able to
change the label.