Some times that happens. Try pulling the battery to see if it returns. If it doesn't try deleting it from Options>Advanced Options>Applications. Then try redownloading it by pointing your phone's browser to www.blackberry.com/messenger
This used to happen to me also. What I found out was going on is that it has to do with Blackberry App World. When I deleted a different app that I downloaded from BB App World using the phone's Options > Applications method, it inadvertently deleted BB Messenger. What I learned was that if you want to delete something you downloaded in BB App World was to delete in within the menu (My World) of BB App World. This method ensures not to delete any other apps downloaded via BB App World then the one you select.
You can also try requesting the service books again. Options>advanced options>host routing table>blackberry key> register now > reboot device.
Registering on the Host Routing Table only registers your device with the data network. It is a good troubleshooting step but it does not send service books to your phone.
To resend service books to your phone you need to click on the Email Settings icon. If you have OS4.7 or higher you can hit your mneu button and select Service Books. If you have 4.5 or lower you need to click on Help then Service Books.