Check this carefully
spn=1000; w=1.093; tareacable=0.005044; initialslack=0.01; strainlimub=0.5;
strainincrement=0.1; slack=initialslack;
iters=(spn*strainlimub-200*initialslack)/(100*strainincrement)+1;
i=1;slacdataStrain=Table[0,{i,1,iters}]; slacdataTension=Table[0,{i,1,iters}];
While[(slack/spn)*100<strainlimub,
slacdataStrain[[i]]= slack/spn*100;
slacdataTension[[i]]=H/.FindRoot[{((2*(H/w)*Sinh[spn*w/(2*H)]-spn-slack)/tareacable)},{H,1000}];
slack=slack+strainincrement;
i++];
points=Transpose[{slacdataStrain,slacdataTension}];
ListPlot[points]
There are a number of warning messages printed which may indicate that no sufficiently precise roots were able to be found for some values of your parameters.