I've been developing a Sharepoint web part, which had been working fine upon deployment. The web part has been developed with WSP Builder, packaged up and then deployed via stsadm. The web part has been deployed tens, if not a hundred times to the dev box with no problems. Now, the web part throws an error which breaks the page it's on:

Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
NYCIRB.DMS.WebParts.SearchUpload.SearchUpload.HandleException(Exception ex) +62
NYCIRB.DMS.WebParts.SearchUpload.SearchUpload.OnLoad(EventArgs e) +214
System.Web.UI.Control.LoadRecursive() +50 System.Web.UI.Control.LoadRecursive() +141 System.Web.UI.Control.LoadRecursive() +141 System.Web.UI.Control.LoadRecursive() +141 System.Web.UI.Control.LoadRecursive() +141 System.Web.UI.Control.LoadRecursive() +141 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627

When looking through my Sharepoint logs, I find these errors repeated over and over which correspond to the time the web part was attempted to be loaded:

01/19/2009 10:53:14.43 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
72kg High (#2: Cannot open "Resources.en-US.resx": no such file or folder.) 01/19/2009 10:53:14.43 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
8e26 Medium Failed to open the language resource for Fea367b94a9-4a15-42ba-b4a2-32420363e018 keyfile Resources. 01/19/2009 10:53:17.55 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
8e25 Medium Failed to look up string with key "XomlUrl", keyfile core. 01/19/2009 10:53:17.55 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
8l3c Medium Localized resource for token 'XomlUrl' could not be found for file with path: "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\Template\Features\Fields\fieldswss.xml".
01/19/2009 10:53:17.55 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
8e25 Medium Failed to look up string with key "RulesUrl", keyfile core. 01/19/2009 10:53:17.55 w3wp.exe (0x05E0)
0x00FC Windows SharePoint Services
General
8l3c Medium Localized resource for token 'RulesUrl' could not be found for file with path: "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\Template\Features\Fields\fieldswss.xml".

I've retracted the web part manually through Solution Management, retracted through stsadm, checked for the existence of the resource file, which is nowhere to be found. I'm pretty much at a loss to why this happened or how to resolve it.