Sharepoint web part stops working because of Resources.en-US.resx file

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.