I also get this problem from time to time. For some reason Maple fails to properly close all opened xml tags. It may be a coincidence of course but I regularly find these failures near (or on) plot output code. One way to bypass it is to find and close these tags. For example I had this issue with one worksheet that displayed the top 50% of the code I had written. I renamed it to worksheet.xml and opened it with Internet Explorer. The xml structure could now be seen and, scrolling down, I found the source of the problem:

The XML page cannot be displayed Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later. -------------------------------------------------------------------------------- The following tags were not closed: Worksheet, Section, Group, Output, Text-field, Plot. Error processing resource 'file://...

Next I opened worksheet.xml in a text editor, closed them manually and saved it with a .mw extension. And lo and behold Maple could now read it again. The obvious downside to this approach is that you lose anything you may have written between the corrupt xml tags (as all you do is close them), but at least the worksheet is working again.

Let me correct something. The system I solve is of the form: Y( i ) = sum( c( i )( j ) * X( i )( j ) + d( i )( j ) ,j=1..nops( X( i ) ) ), that is it's a linear system Ax=b not Ax=0 as my first post implied..

...continued from previous post PS. As the coefficients of X[i]'s in the system include sums and products of X[m]'s (m smaller than i), they tend to be huge. I thought that maybe the size of these expressions had something to do with the decrease in efficiency so I used YY:=Veil[K](Y,X) and then solve(YY,X) to change them to simple symbolic coefficients but that didn't help. In fact it made matters worse because apparently constructing and substituting K takes more time that any time saved (if at all).